'''
使用元类实现 orm 框架. 
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'''
class Field():
    def __init__(self,name,column_type):
        self.name=name
        self.column_type=column_type
    def __str__(self):
        return '<%s:%s>' % (self.__class__.__name__,self.name)

class IntegerField(Field):
    def __init__(self, name):
        super().__init__(name, "bigint")

class StringField(Field):
    def __init__(self, name):
        super().__init__(name, "varchar(100)")

class ModelMetaclass(type):
    def __new__(cls, name, bases, attrs):
        if name=='Model':
            return type.__new__(cls, name, bases, attrs)
        print('Found model: %s' % name)
        mappings = dict()
        print("type(attrs),",type(attrs))
        for k, v in attrs.items():
            if isinstance(v, Field):
                print('Found mapping: %s ==> %s' % (k, v))
                mappings[k] = v
        for k in mappings.keys():
            attrs.pop(k)
        attrs['__mappings__'] = mappings # 保存属性和列的映射关系
        attrs['__table__'] = name  #假设表名和类名一致
        return type.__new__(cls, name, bases, attrs)
    
class Model(dict,metaclass=ModelMetaclass):
    def __init__(self,**kw):
        super(Model,self).__init__(**kw)

    def __getattr__(self, key):
        try:
            return self[key]
        except KeyError:
            raise AttributeError(r"'Model' object has no attribute '%s'" % key)
    def __setattr__(self, key, value):
        self[key] = value       
    def save(self):
        fields = []
        params = []
        args = []
        for k, v in self.__mappings__.items():
            fields.append(v.name)
            params.append('?')
            args.append(getattr(self, k, None))
        sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join(params))
        print('SQL: %s' % sql)
        print('ARGS: %s' % str(args))

class User(Model):
    # 定义类的属性到列的映射：
    id = IntegerField('id')
    name = StringField('username')
    email = StringField('email')
    password = StringField('password')

# 创建一个实例：
u = User(id=12345, name='Michael', email='michael@gmail.com', password='my-pwd')
u.save()





